Increased consumption of communication services fuels a need for increased data carrying capacity or bandwidth in communication systems. A phenomenon known as crosstalk often occurs in these communication systems and can impair high-speed signal transmission and thus limit communication bandwidth to an undesirably low level.
Crosstalk is a condition that arises in communications systems wherein a signal in one communication channel is corrupted by interference (or bleed-over) from a different signal being communicated over another channel. The interference may arise due to a variety of effects. For example, in electrical systems such as circuit boards, electrical connectors, and twisted pair cable bundles, each electrical path serves as a channel. At high communication speeds, these conductive paths behave like antennae, both radiating and receiving electromagnetic energy. The radiated energy from one channel, referred to herein as the “aggressing channel,” is undesirably coupled into or received by another channel, referred to herein as the “victim channel.” This undesirable transfer of signal energy, known as “crosstalk,” can compromise data integrity on the receiving channel. Crosstalk is typically bidirectional in that a single channel can both radiate energy to one or more other channels and receive energy from one or more other channels.
Crosstalk is emerging as a significant barrier to increasing throughput rates of communications systems. When not specifically addressed, crosstalk, often manifests itself as noise. In particular, crosstalk degrades signal quality by increasing uncertainty in the received signal value thereby making reliable communications more difficult, i.e. data errors occur with increased probability. In other words, crosstalk typically becomes more problematic at increased data rates. Not only does crosstalk reduce signal integrity, but additionally, the amount of crosstalk often increases with the bandwidth of the aggressing signal, thereby making higher data rate communications more difficult. This is particularly the case in electrical systems employing binary or multi-level signaling, since the conductive paths over which such signals flow radiate and receive energy more efficiently at the high frequencies associated with the level transitions in these signals. In other words, each signal in a binary or multi-level communication signal is composed of high-frequency signal components that are more susceptible to crosstalk degradation compared to lower frequency components.
The crosstalk impediment to increasing data throughput rates is further compounded by the tendency of the high-frequency content of the victim signal to attenuate heavily over long signal transmission path lengths (e.g. circuit traces that are several inches in length for multi-gigabit per second data rates). That is, high-frequency components of a communication signal not only receive a relatively high level of crosstalk interference, but also are susceptible to interference because they are often weak due to transmission losses.
While these attenuated high-frequency components can be amplified via a technique known as channel equalization, such channel equalization frequently increases noise and crosstalk as a byproduct of amplifying the high-frequency signals that carry data. The amount of crosstalk present in a communication link often limits the level of equalization that can be utilized to restore signal integrity. For example, at the multi-gigabit per second data rates desired for next-generation backplane systems, the level of crosstalk energy on a communication channel can exceed the level of victim signal energy at the high frequencies that underlie such high-speed communication. In this condition, extraneous or stray signal energy can dominate the energy of the desirable data-carrying signals, thus rendering communicating at these data rates impractical with most conventional system architectures.
The term “noise,” as used herein, is distinct from crosstalk and refers to a completely random phenomenon. Crosstalk, in contrast, is a deterministic, but often unknown, parameter. The conventional art includes knowledge that it is theoretically possible to modify a system in order to mitigate crosstalk. In particular, with definitions of: (i) the data communicated over an interfering or aggressing channel; and (ii) the signal transformation that occurs in coupling from the aggressing channel to the victim channel, the crosstalk can be theoretically determined and cancelled. That is, those skilled in the art understand that crosstalk signal degradation can be cancelled if the data carried by a communication signal that is input into a communication channel is known and the signal transformation imposed on the communication signal by crosstalk is also known. However, achieving a level of definition of this signal transformation having sufficient precision and accuracy to support a practical implementation of a system that adequately cancels crosstalk is difficult with conventional technology. Consequently, conventional technology that addresses crosstalk is generally insufficient for high-speed (e.g. multi-gigabit per second) communications systems. Thus, there is a need in the art to cancel-crosstalk so as to improve victim signal fidelity and remove the barrier that crosstalk often poses to increasing data throughput rates.
While the physics giving rise to crosstalk (e.g. electromagnetic coupling in electrical systems or four-wave-mixing in optical systems) is well understood, this understanding alone does not provide direct and simple models for the crosstalk transfer function. One common reason for conventional modeling difficulties is that the relative geometries of the victim and aggressor signal paths heavily influence the transfer function of the crosstalk effect, and these paths can be quite convoluted. In other words, signal path complexity typically checks efforts to model crosstalk using conventional modeling methods based on analyzing signal conduits. Furthermore, it is generally undesirable to design a crosstalk canceller for a predetermined specific crosstalk response since: (i) a system may have many different responses for different victim-aggressor pairs (each requiring a specific design); and (ii) different systems may need different sets of designs. Thus, there is a need in the art for a crosstalk cancellation system and method with sufficient flexibility to: (i) accommodate the variety of crosstalk transfer functions that can stem from ordinary operations of a given system; and (ii) self-calibrate in order to avoid a complex manual task of characterizing and adjusting for each victim-aggressor pair.
While the general concept of crosstalk cancellation is known in the conventional art, conventional crosstalk cancellation is typically not applicable to high-speed environments, such as channels supporting multi-gigabaud rates. Conventional crosstalk cancellation is typically implemented in an entirely digital environment, wherein the accessible aggressing data signals and received victim signals are digitized, and a microprocessor implements the cancellation processes. The analog-to-digital converters and microprocessors needed to implement this digital crosstalk cancellation in a high-speed environment are usually excessively complex, resulting in unacceptable power consumption and product cost.
To address these representative deficiencies in the art, what is needed is a capability for crosstalk cancellation compatible with high-speed environments but with low power consumption and reasonable production cost. A capability is further needed for automatically calibrating or configuring crosstalk cancellation devices. Such capabilities would facilitate higher data rates and improve bandwidth in diverse communication applications.